1. Field of the Invention
The invention relates to a control system for a fluid coupling of an automatic transmission equipped with a lockup clutch, and, more particularly, to a lockup clutch control system for a fluid coupling of an automatic transmission which controls engagement of a lockup clutch during traveling downhill.
2. Description of the Related Art
Typically, a control system for an automatic transmission equipped with a fluid coupling, such as a torque converter, performs a downslope gear shift or slope control which refers to the control which causes a down shift of the automatic transmission from a higher gear, for example a fourth gear, to a lower gear, for example a third gear, to apply engine brake more effectively during traveling on a downslope.
With an automatic transmission equipped with a fluid coupling incorporating a lockup clutch, when the lockup clutch is changed in operation from a disengaged mode in which the fluid coupling is unlocked to allow relative rotation between a pump as a rotation input element and a turbine as a rotation output element of the fluid coupling to a full-engagement mode in which the pump and turbine of the fluid coupling is locked together or a slip mode in which the lockup clutch is allowed to slip for a gear shift during traveling downhill, deceleration is felt by the driver as a drawing shock due to overlapped shocks resulting from the change in operation mode of the lockup clutch and a down shift and a feeling of deceleration is intensified due to increased reverse torque transmitted to the engine from driving wheels.
There have been proposed on-slope gear shift control systems which cause a gear shift according to downslopes and upslopes. Such an on-slope gear shift control system is known from, for example, Japanese Unexamined Patent Publication No. 5-716262. Lockup clutch slip control has been known in various forms one of which is described in, for example, Japanese Unexamined Patent Publication No. 3-13060. However, there has been neither discussion regarding an enhanced and unpleasant feeling of deceleration occurring in an automatic transmission coupled with these features nor suggestion regarding how to relieve the enhanced and unpleasant feeling of deceleration.